1. Field of the Invention
The present invention relates to a superconducting composite and a process for preparing the same, more particularly, it relates to a superconducting composite which can make good use of superconductivity which exhibits a higher critical temperature and a process for producing the same.
2. Description of the Related Art
Under the superconducting condition, the perfect aliamagnetism is observed and no difference in potential is observed for all that an electric current of a constant finite value is observed internally, and hence, a variety of applications of superconductivity have been proposed in a field of electric power transmission as a mean for delivering electric power without loss.
The superconductivity can be utilized in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation for example magnetic levitation trains, or magnetically propelling ships; in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, or in the field of fusion power generation.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, their actual usage have been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2K at most. This means that liquidized helium (boiling point of 4.2K) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited cosily resource but also require a large-scaled system for liquefaction. Therefore, there had been a strong demand for another superconducting materials having higher Tc. But no material which exceeded the abovementioned Tc had been found for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986)189]
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 which are called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30 to 50K which are extremely higher than the known superconducting materials and hence it becomes possible to use liquidized hydrogen (b.p.=20.4K) or liquidized neon (b.p.=27.3K) as a cryogen which brag them to exhibit the superconductivity.
It was also reported in the news pater that C. W. Chu et al discovered in the Untied States of America another type of superconducting material having the critical temperature of in the order of 90K in February 1987, and hence possibility of existence of high-temperature superconductors have burst on the scene.
However, the above mentioned new type superconducting materials which was just born have been studied and developed only in a form of sintered bodies as a bulk produced from powders and hence is very fragile, so that careful handling is requested. In fact, they are apt to break or crack easily under mechanical stress, particularly, when they are shaped into a wire form.
It is difficult to produce a slatered body composed of completely uniform polycrystal possessing higher superconductivity from particles having superconducting properties alone. Still more, the superconductivity is apt to be easily broken in stronger magnetic field or fluctuation and/or unhomogeneous distribution of temperature in the sintered body.
The abovementioned oxide type superconducting materials possess rather higher specific resistance and lower heat-conductivity above the critical temperature. Therefore, if the phenomenon of superconductivity breaks locally, the sintered body generates locally Joule heat caused by the superconducting current and explosive evaporation of cryogen is induced when the heated portion of the sintered body contacts with the cryogen. In order to avoid this trouble, in conventional metal type superconducting material, superconducting metal is shaped in a form of a fine wire and a plurality of which are embedded in electroconductive material which play a roll of a by-pass of electric current.
The oxide type superconducting materials are, however, difficult to be shaped or deformed into such filaments, because they have not superior plasticity or processability in comparison with well-known metal type superconducting materials such as Ni-Ti alloy.
Therefore, an object of the present invention is to overcome the abovementioned problems and to provide a new superconducting composite which can be used under more stable conditions safely and a process for producing the same.